Caffeine melting point

Caffeine (melting point 238°C, sublimes at 178°C density 1.23), theobromine, and theophylline are xanthine derivatives classified as central nervous stimulants, but differing markedly in their properties. They can be extracted from a number of natural sources. 

Sublimation is a process that involves the direct conversion of a solid to a gas without passing through the liquid state. Relatively few solids do this at atmospheric pressure. Some examples are the solid compounds naphthalene (mothballs), caffeine, iodine, and solid carbon dioxide (commercial Dry Ice). Water, on the other hand, sublimes at — 10°C and at 0.001 atm. Sublimation temperatures are not as easily obtained as melting points or boiling points. 

Take a melting point of your solid. First, scrape the caffeine from the bottom and sides of the flask with a microspatula and collect a sample of the solid in a capillary tube (review Experiment 15 for the technique). Pure caffeine melts at 238°C. Compare your melting point (6) to the literature value. 

Caffeine forms white hexagonal crystals by sublimation. Caffeine has a melting point of 238 Celsius, but the crystals begin to sublime when heated to 178 Celsius. Caffeine is only moderately soluble in water, but more soluble in hot water. The crystals are also moderately soluble in alcohol, acetone, but are much more soluble in methylene chloride, chloroform, and practically insoluble in ether. Caffeine is capable of forming a hydrate, which looses it water of hydration when heated to 80 Celsius. Caffeine is a widely used stimulant, ingested by millions in the form of coffee, tea, ect.,... 

One way to confirm the identity of an organic compound is to prepcne a derivative of it. Caffeine melts and sublimes at 238°C. It is an organic base and can therefore accept a proton from an acid to form a salt. The salt formed when caffeine combines with hydrochloric acid, like many amine salts, does not have a sharp melting point it simply decomposes when heated. But the salt formed from scilicyclic acid, even though ionic, has a sharp melting point and can thus be used to help characterize caffeine. 

To 50 mg of sublimed caffeine in a tared test tube add 38 mg of scilicyclic acid and 2.5 mL of dichloromethane. Heat the mixture to boiling and add petroleum ether (a poor solvent for the product) dropwise until the mixture just turns cloudy, indicating the solution is saturated. If too much petroleum ether is added then clarify it by adding a very small quantity of dichloromethane. Insulate the tube in order to allow it to cool slowly to room temperature, and then cool it in ice. The needle-like crystcils are isolated by removing the solvent while the reaction tube is in the ice bath. Evaporate the last traces of solvent under vacuum and determine the weight of the derivative and its melting point. Caffeine salicylate is reported to melt at 137°C. 

The caffeine can be purified by sublimation as has been done in the experiment in which it was extracted from tea (Fig. 2) or it can be purified by crystallization. Recrystallize the caffeine by dissolving it in the minimum quantity of 30% ethanol in tetrahydrofuran. It can also be crystallized by dissolving the product in a minimum quantity of hot toluene or acetone and adding to this solution ligroin (hexanes) until the solution is cloudy while at the boiling point. In any case allow the solution to cool slowly to room temperature, then cool the mixture in ice, and remove the solvent from the crystals with a Pasteur pipette. Remove the remainder of the solvent under aspirator vacuum, and determine the weight of the caffeine and its melting point. 

Modification of the solvent of crystallisation may result in different solvated forms. This is of particular relevance because the hydrated and anhydrous forms of a dmg can have melting points and solubilities sufficiently different to affect their pharmaceutical behaviour. For example, glutethimide exists in both an anhydrous form (m.p. 83°C, solubility 0.042% at 25°C) and a hydrated form (m.p. 68°C, solubility 0.026% at 25°C). Other anhydrous forms show similar higher solubilities than the hydrated materials and, as expected, the anhydrous forms of caffeine, theophylline, glutethimide and cholesterol show correspondingly higher dissolution rates than their hydrates. 

Ternary eutectics are also possible. The binary eutectic points of three mixtures are as follows for aminophenazone-phenacetin 82°C for aminophenazone-caffeine 103.5°C and for phenacetin-caffeine IZS C. The ternary eutectic temperature of aminophenazone-phenacetin-caffeine is 81°C. In this mixture the presence of aminophenazone and phenacetin can be detected by the mixed melting point... 

The caffeine obtained from tea or coffee can be purified by sublimation using the procedure described in Experiment 13A. At your instructor s option, you may combine your sample with another student s sample for sublimation. After sublimation, determine the weight of caffeine recovered and calculate the weight percentage recovery of the caffeine. Compare this value to the amount of crude sample obtained. At the instructor s option, determine the melting point of the purified caffeine. The melting point of pure caffeine is 236°C however, the observed melting point will be lower. Submit the sample to the instructor in a labeled vial unless it is to be used for infrared spectroscopy (recommended) or mass spectroscopy (also recommended). 

Attach your infrared spectra to your report and label the major peaks. If you determined the mass spectrum, try to identify the important fragment ion peaks (Technique 28). Include the melting point, if it was required. Report the weight percentage of the caffeine recovered from the tea or coffee sample before and after sublimation. 

What are some possible explanations for why the melting point of your isolated caffeine was lower than the literature value (236°C) ... 

Caffeine, a stimulant found in coffee, tea, mate, and cola drinks, is a purine. It is a colorless, odorless substance with melting point 236°C. Its... 

According to Kofler [358], microsublimation is any sublimation procedure in which the sublimate is caught on a smooth plate-usually a small slide-for purposes of microscopic examination (melting point, refraction, crystal structure). Many substances can be directly sublimed in crystalline form from drugs containing them, e. g., caffeine coumarin umbelliferone benzoic, ferulic, salicylic and cinnamic acids emodin etc. 

Now determine an evacuated melting point (see pp. 51-52 for the experimental details on how to proceed with this measurement) of caffeine and compare it to the values in the CRC and Aldrich references. Your instructor will provide you with the experimental details concerning the operation of the particular instrument to be used in your laboratory. 

NOTE. Be careful. Do not melt the caffeine. If the sample does begin to melt, remove the flame for a few seconds before heating is resumed. Overheating the crude sample will lead to decomposition and the deposition of impurities on the cold finger. High temperatures are not necessary since the sublimation temperature of caffeine (and of all solids that sublime) is below the melting point. It is generally worthwhile to carry out sublimations as slowly as possible, as the purity of the material collected will be enhanced. 

If your melting point apparatus uses capillary tubes to determine the melting point, an evacuated sealed tube is necessary, since caffeine sublimes the melting point is above the sublimation temperature (see Chapter 4). The melting point may be obtained using the Fisher-Johns apparatus without this precaution. 

Purification and Characterization. The product normally is suffidently pure for direct characterization. Weigh the caffeine 5-nitrosalicylate and calculate the percent yield. Determine the melting point and compare your value with that reported above. 

First, their water solubility clearly increases according to the series xanthine, theobromine, theophylline, and caffeine. Curiously, the most methylated derivative is the most soluble and the least methylated one the least soluble. Second, no regularity is found in the melting-point values of these derivatives. The explanation of these anomalies lies, at least in part, in the formation of hydrogen bonds. In caffeine, the... 

The relative affinity of the studied solutes to ethyl lactate follows the order thymol feruiic acid > vanillic acid > caffeine > caffeic acid. As expected, solubilities of all studied solutes in ethyl lactate were moderately enhanced by temperature rise. Thymol is extremely soluble in ethyl lactate, reaching ca. 91 wt% at 317.8K which can be explained by its relatively low melting point and enthalpy of fusion. 

---